The invention pertains to a light source used for open-path gas monitoring, particularly for the measurement of the smoke and dust content of stack gases, but also applicable to the measurement of particulates in the atmosphere.
The standard method for continuous emissions measurement of particulates in stacks and ducts is optical transmissometry. The measured quantity is opacity, defined as the fraction of transmitted light which is lost in transmission through a medium.
One example of a device that measures opacity, known as a transmissometer, is the Land Combustion Model 4500mkII opacity monitor which has been used for a number of years to measure the opacity of gases in stacks and ducts. A functional diagram of the Model 4500mkII is shown in FIG. 1 wherein the Model 4500mkII consists of two main units: a transceiver 20 mounted on one side of a stack/duct 22 and a passive retroreflector 24 mounted on the other side. A light source LS in the transceiver 20 projects a beam of light 26 along the transceiver""s optical axis 27 across the duct 22, through the dust/smoke in the open path 28 of the gas/smoke 29 (FIG. 2) to the retroreflector 24 which returns a reflected light beam 30 to an analyzer A in the transceiver 20. The analyzer A then compares the intensity of the returned radiation with that measured under clear-stack conditions in order to calculate the opacity and then displays this opacity value at a remote location (e.g., a data recorder, not shown). Also see U.S. Pat. No. 5,617,212 (Stuart), whose entire disclosure is incorporated by reference herein, for a detailed description of how the analyzer A calculates the opacity.
FIG. 2 shows the Model 4500mkII mounted to the stack/duct 22 and depicts the internals of the transceiver 20. In particular, the light source LS of the transceiver 20 comprises an LED (light emitting diode) 32. The transceiver 20 also comprises a beamsplitter 34, a collimating lens 36, a folding mirror 38, and the analyzer A which comprises a measurement detector 40, a reference detector 42 and a processor 43 (e.g., Hitachi H8/500 microprocessor). Additional components include a flood LED 44 for drift correction, an automatic zero 46 and span 48 devices and a fail-safe shutter 50. It should be understood that the transmissometer is autocollimated meaning that the return light 30 from the retroreflector 24 is along the same path as the projected beam 26. External electrical power (e.g., 110VAC @ 60 Hz), not shown, is provided to the transceiver 20 for energizing the electrical components.
The divergence 52 of the projected light beam 26 means that the retroreflector 24 returns only a portion of the projected light 26. Any change in alignment, (e.g., because of temperature changes, wind, settling, etc.) in the stack/duct 22 walls, results in a different portion of the projected beam 26 falling on the retroreflector 24. Moreover, because the projected beam 26 is not perfectly homogeneous, i.e., the light intensity varies across the projected beam (see line 54), a change in alignment results in a change in light intensity. This change is wrongly interpreted by the analyzer A as a change in the opacity of the stack/duct 22 gases.
Errors are also introduced where an opacity monitor (transmissometer) with an inhomogeneous light beam is calibrated in the laboratory and then installed on the stack/duct 22. In this case, failure to perfectly reproduce the device""s optical alignment between the laboratory and the duct results in a signal offset. This offset is, in many cases, the dominant source of error in the measurement. As a consequence, the detection limit of the opacity monitor may be set by this offset.
A number of factors affect the homogeneity of the projected beam 26, including the precision and cleanliness of the optical components used. However, the principal factor is usually the inhomogeneity of the light source LS. There are a number of factors which make the pattern of light from an LED inhomogeneous. Some of these are symmetrical about the optical axis of the LED and some are not. This is especially so when a LED source is used, since the electrical contact to the center of the die results in a dark spot in the middle of the beam 26.
One way of producing a homogeneous light source is to use an integrating sphere, such as that described in xe2x80x9cA Guide to Integrating Sphere Theory and Applicationsxe2x80x9d by Labsphere. However, an integrating sphere is both bulkier and more expensive than the present invention.
The limitations of the present state of the art are reflected in ASTM (American Society for Testing and Materials) Standard Practice for Opacity Monitor Manufacturers to Certify Conformance with Design and Performance Specifications D6216-98 (1998) which is incorporated by reference into U.S. 40 C.F.R. xc2xa760, Appendix B, EPA Performance Specification 1, and which concerns the use of opacity monitors for regulatory applications at opacity levels of 10% or higher. However, where detecting opacity levels of less than 10% is important, e.g., in the steel industry, no performance specification currently exists for the use of opacity monitors to ensure compliance with opacity limits below 10%.
Thus, there remains a need for a transmissometer that can minimize the inhomogeneity of the light source and can therefore detect opacity levels below 10% while operating within specific performance requirements.
A light source for use in an opacity monitor for measuring the opacity of gases in an open path of gases wherein the light source reduces the variation in light intensity across a beam of light projected therefrom.
A light source adapted for use in open path gas monitoring wherein the light source generates a homogeneous light beam.
An opacity monitor for measuring the opacity of gases in an open path of gases wherein opacity is defined as the fraction of transmitted light which is lost in transmission through the open path of gases. The opacity monitor comprises: an optical transmitter for projecting a light beam across the open path of gases using a light source that reduces the variation in light intensity across the projected beam; a reflector for reflecting a portion of the projected light beam back towards the optical transmitter through the open path gas of gases; an analyzer for detecting the portion of the projected light beam and calculating the opacity of the gases; and wherein the optical monitor detects opacities less than 10 percent while operating within specific performance requirements (e.g., all the requirements of ASTM D6216-98, including amendments to specific portions of ASTM D6216-98 to ensure compliance with opacity limits below 10%, such as thermal stability, insensitivity to ambient light, zero and span calibration, measurement of output resolution, calibration error, optical alignment indicator, calibration device value and repeatability, and insensitivity to supply voltage variations).
An opacity monitor for measuring the opacity of gases in an open path of gases wherein opacity is defined as the fraction of transmitted light which is lost in transmission through the open path of gases. The opacity monitor comprises: an optical transmitter having a light source that projects a homogeneous light beam across the open path of gases; a reflector for reflecting a portion of the projected homogeneous light beam back towards the optical transmitter through the open path gas of gases; an analyzer for detecting the portion of the projected homogeneous light beam and calculating the opacity of the gases; and wherein the optical monitor detects opacities less than 10 percent while operating within specific performance requirements(e.g., all the requirements of ASTM D6216-98, including amendments to specific portions of ASTM D6216-98 to ensure compliance with opacity limits below 10%, such as thermal stability, insensitivity to ambient light, zero and span calibration, measurement of output resolution, calibration error, optical alignment indicator, calibration device value and repeatability, and insensitivity to supply voltage variations).
A method for reducing the variation in light intensity across a beam of light projected from a light source used in an opacity monitor. The method comprises the steps of: (a) providing a plurality of light emitting diodes (LEDs), each having a respective optical axis and each emitting respective light beams; (b) arranging the plurality of LEDs at a predetermined angular orientation with respect to each other and aligning each of the optical axes to be parallel to each other; and (c) positioning an optical diffuser at a predetermined distance away from the plurality of LEDs for mixing and reflecting the respective light beams to form the beam of light having a reduced variation in light intensity.
A method for reducing the variation in light intensity across a beam of light projected from a light source used in an opacity monitor. The method comprises the steps of: (a) providing a plurality of light emitting diodes (LEDs), each having a respective optical axis and each having symmetrical and asymmetrical inhomogeneities in respective light beams emanating from each LED; (b) minimizing the symmetrical and asymmetrical inhomogeneities in the respective light beams by: (1) orienting the plurality of LEDs within in a common plane; and (2) positioning an optical diffuser at a predetermined distance away from the plurality of LEDs to mix and reflect the respective light beams to form the beam of light having the reduced variation in light intensity across the beam of light.
A method for measuring the opacity of gases in an open path of gases wherein opacity is defined as the fraction of transmitted light which is lost in transmission through the open path of gases. The method comprises the steps of: (a) projecting a light beam across the open path of gases using a light source that reduces the variation in light intensity across the projected beam; (b) reflecting a portion of the projected light beam; (c) detecting and analyzing the portion of the portion of the projected light beam; (d) detecting opacities less than 10 percent while operating within specific performance requirements (e.g., all the requirements of ASTM D6216-98, including amendments to specific portions of ASTM D6216-98 to ensure compliance with opacity limits below 10%, such as thermal stability, insensitivity to ambient light, zero and span calibration, measurement of output resolution, calibration error, optical alignment indicator, calibration device value and repeatability, and insensitivity to supply voltage variations).